Port in use?

I've been hitting some issues with irc. Simply put I can't access any irc servers on port 6667. No matter if I use irc programs (such as mIRC), the Trillian client or Java clients, I get the message "software caused connection about". Servers running on other ports I can access.

I'm running XP Pro SP2 with the stanard XP firewall. I've tried making exceptions and turning the firewall off, without effect, so I'm assuming that another program is already using the port. Am I likely to be right and how would I find out?

Thanks in advance